1. Field of the Invention
The present invention relates to a lithographic apparatus, and a device manufacturing method.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
The imaging of the pattern involves illuminating the patterning device with electro magnetic radiation. With high Numerical Aperture (NA) projection systems and with high NA immersion projection systems for transfer of the pattern, it is desirable to provide polarized or at least partially polarized illumination radiation. This enables image formation at wafer level by radiation with a state of polarization which is suitable for optimal contrast of the image. For example, where image forming radiation has a p-polarized, an s-polarized and an unpolarized component, it is in particular the s-polarized radiation component which contributes most to contrast of the image. Consequently an illumination system for use with a lithographic apparatus may therefore be arranged to specifically provide s-polarized illumination radiation. However, optical elements downstream of the illumination system may destroy or affect the state polarization of radiation traversing these elements due to, for example, the presence of residual or inherent optical birefringence in the material of the elements or due to effects of optical (single or multi-layer) coatings on the element surfaces. During assembly of an optical system of a lithographic apparatus parameters affecting a state of polarization can be monitored and tolerance-controlled. However, optical elements not being a fixed part of the optical system, such as for example a patterning device embodied as a reticle, may cause a depolarization or a change of polarization affecting image contrast beyond tolerance.